In a magnetic recording device in which a read head is based on a spin valve magnetoresistance (SVMR) or a giant magnetoresistance (GMR) effect, there is a constant drive to increase recording density. One method of accomplishing this objective is to decrease the size of the sensor element in the read head that is suspended over a magnetic disk on an air bearing surface (ABS). The sensor is a critical component in which different magnetic states are detected by passing a sense current through the sensor and monitoring a resistance change. A GMR configuration includes two ferromagnetic layers which are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields. In the absence of an external magnetic field, the magnetization direction of the free layer is aligned perpendicular to that of the pinned layer by the influence of hard bias layers on opposite sides of the sensor stack. When an external magnetic field is applied by passing the sensor over a recording medium at the ABS, the free layer magnetic moment may rotate to an opposite direction. Alternatively, in a tunneling magnetoresistive (TMR) sensor, the two ferromagnetic layers are separated by a thin non-magnetic dielectric layer.
A sense current is used to detect a resistance value which is lower in a (0) memory state than in a (1) memory state. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the sensor stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the sensor layers.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head in which the cross-sectional area of the sensor is typically smaller than 0.1×0.1 microns at the ABS plane. Current recording head applications are typically based on an abutting junction configuration in which a hard bias layer is formed adjacent to each side of a free layer in a GMR spin valve structure. As the recording density further increases and track width decreases, the junction edge stability becomes more important so that edge demagnification in the free layer needs to be reduced. In other words, horizontal (longitudinal) biasing is necessary so that a single domain magnetization state in the free layer will be stable against all reasonable perturbations while the sensor maintains relatively high signal sensitivity.
In longitudinal biasing read head design, hard bias films of high coercivity are abutted against the edges of the sensor and particularly against the sides of the free layer. In other designs, there is a thin seed layer between the hard bias layer and free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjoining hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for sensor elements become smaller with higher recording density requirements, the free layer becomes more volatile and more difficult to bias which means the minimum longitudinal bias field necessary for free layer domain stabilization must be increased.
Imperfect alignment with a hard magnetic thin film such as a free layer can give rise to hysteresis or non-linear response of the sensor and thus noise. In general, it is desirable to enhance the coercivity of the hard bias film so that the stray field created by the recording medium will not destroy the magnetic alignment of the free layer. A high coercivity in the in-plane direction is needed in the hard bias layer to provide a stable longitudinal bias that maintains a single domain state in the free layer and thereby avoids undesirable Barkhausen noise. This condition is realized when there is a sufficient in-plane remnant magnetization (Mr) from the hard bias layer which may also be expressed as Mrt since hard bias field is also dependent on the thickness (t) of the hard bias layer. Mrt is the component that provides the longitudinal bias flux to the free layer and must be high enough to assure a single magnetic domain in the free layer but not so high as to prevent the magnetic field in the free layer from rotating under the influence of a reasonably sized external magnetic field. Moreover, a high squareness (S) hard bias material is desired. In other words, S=Mr/MS should approach 1 where MS represents the magnetic saturation value of the hard bias material. A higher Mr will advantageously allow t to be smaller for advanced designs with high recording density. p Many efforts have been made to achieve higher coercivity in hard bias films and the improvement is generally realized through altering the composition of an underlying seed layer. U.S. Patent Appl. Publication 2008/0151441 describes a Ta or Cr seed layer that is ion milled to form an anisotropic surface texture under the hard bias layer to increase coercivity. In U.S. Patent Appl. Publication 2002/0015268, a composite seed layer having a Cr/TiW stack with a 50 Angstrom thickness for each layer is employed to raise Hc for an overlying CoPt hard bias layer to 2300 Oe from 1800 Oe with a conventional Cr seed layer/CoPt hard bias layer configuration. Similarly, U.S. Patent Appl. Publication 2006/0087772 discloses a NiTa (10 A)/CrMo (40 A) seed layer that increases coercivity of an overlying CoPt hard bias layer to 2000 Oe. It should be understood that the magnetostatic coupling provided by a hard bias layer drops off quickly with increased spacing between the hard bias layer and free layer. In addition, it is important not to adjust the thickness of the hard bias layer since the thickness value is more or less set based on the design requirement for the sensor. Changing the thickness of the hard bias layer might require adjusting the thickness of one or more layers in the sensor which could degrade performance. Therefore, an improved hard bias structure is needed in which the coercivity of the hard bias layer can be increased by using existing materials and without changing the thickness of the seed layer and hard bias layer.